1. Technical Field
The technical field is circuit components. More particularly, the technical field includes powders and pastes used to form dielectric and conductive elements.
2. Related Art
Passive components may be embedded in printed wiring board innerlayer panels that are stacked and connected by interconnection circuitry, the stack of panels forming a printed wiring board. Embedded capacitors are subject to requirements such as acceptable capacitance density, low dielectric loss, high breakdown voltage, and good stability of capacitance within specified temperature ranges. For example, Electrical Industry Association designation Z5U requires that a capacitor's capacitance vary by not more than +/−22% over the temperature range of 10-85° C., and Electrical Industry Association designation Y5V requires a dissipation factor (Df) of less than 3%. The physical and electrical properties of embedded components are largely dependent on the materials used to form dielectric elements, conductive elements, and other elements of the components.
Barium titanate is commonly selected as the base material for pastes used to form high capacitance thick-film dielectrics. In components such as capacitors, high dielectric constants (K) for dielectric layers are desirable because they allow for smaller capacitor size. Pure barium titanate has its maximum capacitance at its Curie point, which is at 125° C., making pure barium titanate unsuitable for many applications. The addition of dopants, however, combined with high temperature processing, is a common method for shifting the Curie point of barium titanate-based materials. Specific amounts and/or chemistries of dopants may be chosen to place the Curie point where it is desired, such as at 25° C., so that the capacitance at room or near temperature is maximized.
Conventional dopants such as barium zirconate, niobium oxide, and strontium titanate may not be suitable for all applications, such as firing at the lower temperatures used in thick-film processing. For example, conventional multilayer ceramic capacitors with such dopants are typically sintered for two (2) hours or more in air or in reducing atmospheres, at peak temperatures in the vicinity of 1100° C. to 1400° C. The conventional dopants are not effective for fired-on-foil applications performed using nitrogen-based thick-film firing profiles of shorter duration and lower temperatures.
High capacitance thick-film dielectric materials such as pastes are further constrained by the requirement of sintering aids, which must be added to barium titanate in order to form a well-sintered dielectric. Conventional sintering aid glasses such as lead boro-silicates, however, have lower dielectric constants and their inclusion lowers the dielectric constant of the resulting composite. The level of glass required for conventional formation of a well-sintered dielectric often results in very low dielectric constants.
Conductive pastes are used to form the capacitor electrodes of fired-on-foil capacitors. Thick-film conductive pastes typically have a metal powder component and a glass powder component dispersed in an organic vehicle. During firing, the metal powder sinters together and the glass forms a bond with the substrate. Conventional conductive pastes that are fired on substrates of materials such as alumina are designed for conductor properties, and not for electrode properties. Therefore, the pastes are generally thicker than is desirable for capacitor electrodes and contain glasses that are not chemically or physically compatible with a barium titanate-based dielectric.